1. Field of the Invention
The present invention relates to an imaging apparatus and a control method therefor and, more particularly, to a technique of reducing image artifacts upon imaging an object by irradiating it with radiation.
2. Description of the Related Art
In recent years, in the field of digital X-ray imaging apparatuses, a large-area flat panel type radiation imaging apparatus of a non-scaled optical system using photoelectric conversion elements has been widely used, instead of an image intensifier, for the purpose of increasing the resolution, decreasing the volume, and suppressing image distortion.
As a flat panel sensor of a non-scaled optical system which is used for a radiation imaging apparatus, there is available a large-area flat panel sensor formed by two-dimensionally joining photoelectric conversion elements formed on a silicon semiconductor wafer by a CMOS semiconductor manufacturing process. Japanese Patent Laid-Open No. 2002-026302 describes a method of manufacturing a large-area flat panel sensor by tiling a plurality of rectangular semiconductor substrates which are strip-shaped rectangular imaging elements obtained by cutting photoelectric conversion elements from a silicon semiconductor wafer. This method can obtain an imaging region for a large-area flat panel sensor, which is equal to or larger than the size of the silicon semiconductor wafer.
Japanese Patent Laid-Open No. 2002-344809 describes the circuit arrangement of each strip-shaped rectangular semiconductor substrate obtained by cutting photoelectric conversion elements. On each strip-shaped rectangular semiconductor substrate, two-dimensionally arrayed photoelectric conversion elements and vertical and horizontal shift registers as readout control circuits are arranged. External terminals (electrode pads) are provided near the horizontal shift register. Control signals and clock signals input from the external terminals control the vertical and horizontal shift registers on the rectangular semiconductor substrate to sequentially output values of the respective pixel arrays from the respective shift registers in synchronism with the clock signals.
Japanese Patent Laid-Open No. 2002-345797 describes an arrangement in which a sampling operation associated with photoelectric conversion is simultaneously performed for all pixels in a flat panel sensor formed by tiling a plurality of rectangular semiconductor substrates. This arrangement implements a collective electronic shutter and equalizes the accumulation times of the respective pixels, thereby preventing pixel value discontinuity that might be caused by the tiling of the rectangular semiconductor substrates.
It is well known, however, that semiconductors such as rectangular semiconductor substrates, A/D converters, and differential amplifiers which form a flat panel sensor as described above generally generate shot noise, thermal noise, and 1/f (flicker) noise. Especially in a semiconductor manufactured by a MOS process, 1/f noise is predominant in the low-frequency region. In the flat panel sensor formed by tiling the plurality of rectangular semiconductor substrates, 1/f noise is superimposed on digital image data for each block A/D converted by an A/D converter. Therefore, block-shaped artifacts occur in the captured image of the flat panel sensor.
In, for example, a dark image which has undergone FPN (fixed pattern noise) correction and has not been exposed to radiation, if no 1/f noise occurs, a very flat image shown in FIG. 11A is obtained. If, however, low-frequency 1/f noise occurs in each of the rectangular semiconductor substrates, differential amplifiers, and A/D converters, a block-shaped artifact for each A/D converter appears, as shown in FIG. 11B. Especially, the radiation imaging apparatus needs a wide dynamic range, and thus low noise is required for a readout circuit used for the radiation imaging apparatus.